I believe OAuth doesn’t require JWT (just an opaque token, which in practice is often JWT), but OpenID Connect – which is based on OAuth – does require JWT.
Some optional OAuth extension RFCs do depend on JWT, e.g. Profile for OAuth 2.0 Access Tokens (RFC 9068) OAuth 2.0 Demonstrating Proof of Possession (DPoP) (RFC 9449, JWT-Secured Authorization Request (JAR) (RFC 9101). Core OAuth 2.0 does not enforce supporting JWT anywhere, but due to the influence of OpenID Connect there are more and more OAuth use cases that require JWT if you want to follow standards beyond the core OAuth RFCs (6749 and 6750).
The closest OAuth gets to mandating JWT is with client authentication and proof-of-possession. The OAuth Best Current Practices RFC (9700) recommends using asymmetric JWT for client authentication in case you cannot use Mutual TLS (which is usually the case). This recommendation will probably be rolled into the new OAuth 2.1 standard (it is included in the draft). OAuth 2.1 also mentions the JWT-based DPoP as one of the two recommended methods for implementing sender-constrained access tokens (the other one is Mutual TLS again).
